Risks of a nuclear war are growing [21, 22]. No nuclear-armed state is currently disarming, nor engaged in nuclear disarmament negotiations. First the US, followed by Russia, abrogated hard-won treaties negotiated between them which were fruits of the end of the first Cold War, and which constrained nuclear weapons numbers and types. Together these two countries hold 90% of all nuclear weapons . The treaties include the Anti-Ballistic Missile Treaty, the Intermediate Nuclear Forces Treaty (eliminated short and medium-range nuclear missiles from the Soviet, then from Russian and US arsenals), the Open Skies Treaty (increased nuclear transparency), and the more recent Joint Comprehensive Plan of Action (Iran nuclear deal that provided effective constraints on Iran's nuclear program until the Trump administration abrogated it). Were it not for the incoming Biden administration's quick agreement to extend the New START treaty just two days before it would otherwise have expired, there would be no treaty restraints in force in 2021 on US and Russian nuclear weapons despite an effectively resurgent Cold War.
Modernising and expanding nuclear arsenals at enormous and escalating cost
All nine nuclear-armed states are investing massively in modernising and expanding their nuclear arsenals. Modernisation means new, faster, stealthier, more flexible and accurate capacities. A number can be armed with either conventional or nuclear warheads, indistinguishable until point of impact. These changes lower the overall threshold for use of nuclear weapons . Both Russia and the USA, owning between them 90% of all nuclear weapons, are comprehensively replacing and modernising their warheads, missiles and launch platforms. They are also increasing the role of nuclear weapons in their military policies, and the range of circumstances in which they might be used, including against conventional and cyber attacks [24, 25]. Russia is testing and deploying entirely new types of nuclear weapons including nuclear-powered cruise missiles, hypersonic delivery vehicles atop ballistic missiles, and long-range nuclear torpedoes designed to explode in waters close to cities . The US is producing new nuclear warheads for the first time in three decades, modernising all types of nuclear weapons—ballistic and cruise missiles, bombs delivered by aircraft, and the submarines, ships and aircraft that carry them . It is also upgrading the nuclear weapons it provides to the UK and the nuclear bombs it stations in Belgium, Germany, Italy, Netherlands and Turkey .
Current estimates of global spending on development and production of nuclear weapons reached US$72.6 billion in 2020, an increase of $1.4 billion from 2019, even given constraints of the pandemic . The total cost of nuclear weapons programs, including environmental clean-up and legacy costs, is far greater. The US spends the most on military and nuclear weapons: in FY 2021 its nuclear weapons-related costs reached US$74.75 billion . Military spending consumes half of all discretionary US government spending. In the US, nuclear warhead spending is currently at an all-time record high, with projected expenditures over the next three decades of over US$2 trillion to comprehensively refurbish the nuclear arsenal and the facilities that produce nuclear weapons . While Russia's military spending in 2020 ($61.7 billion) was estimated to be only 8% of that of the US ($778 billion) , the proportion it spends on nuclear weapons is more than 2.5 times as great as the US .
Opportunity costs: weapons versus United Nations and related programs
Such vast expenditures on weapons that create a hazardous legacy even in their production have enormous social, environmental, and public health opportunity costs. Estimates by the Sustainable Development Solutions Network place average total annual investment required between 2019 and 2030 to fully finance achievement of the Sustainable Development Goals (SDGs) agreed by all nations at US$1011 billion . That amounts to about half of annual military expenditures, US$1981 billion in 2020—2.6% higher than in 2019 . That increase occurred despite the COVID-19 pandemic and the associated severe economic downturn, increase in poverty and food insecurity. The combined annual budgets of the World Health Organization (WHO), UNICEF, the United Nations itself, the UN High Commissioner for Refugees, the International Committee of the Red Cross, and the UN Office of Disarmament Affairs amount to less than 30% of direct spending on nuclear weapons . Operating an F-35 nuclear-capable combat aircraft for one-hour costs as much as a nurse earns in a year (OECD average); the cost of one Virginia Class nuclear submarine could fund 9180 fully equipped ambulances; the cost of one Trident II nuclear missile could buy 17 million facemasks . By September 2021, at least one dose of COVID-19 vaccine had reached fewer than 3% of people in low-income countries and the WHO fell short by US$900 million in funds they needed to cover the period till March 2022 for their essential role in ending the acute phase of the pandemic—1.2% of annual direct nuclear weapons spending .
Doomsday clock reflects growing insecurity
Leaders of all nuclear-armed states have, in recent years, issued specific nuclear threats, with military leaders confirming their active planning to fight nuclear war . In 2020, the Bulletin of the Atomic Scientists moved its authoritative Doomsday Clock to 100 s to midnight, further forward than it has ever been before, explaining that: "the international security situation is now more dangerous than it has ever been, even at the height of the Cold War."  In 2021 the clock hands remained in the same position, as: "the potential to stumble into nuclear war—ever present—has grown."  In 2019, the United States intelligence community's annual assessment to Congress of worldwide threats warned that the effects of climate change and environmental degradation increase stress on communities around the world and intensify global instability and the likelihood of conflict, increasing the danger of nuclear war . Over the last decade, the number of armed conflicts has steadily grown, particularly the number of "internationalised intrastate" conflicts—within a state but involving at least one nation (disproportionately nuclear-armed nations) outside the state in conflict .
Cyberwarfare increases vulnerability of nuclear arms systems
Another major area of increasing risk of use of nuclear weapons is growing use of cyberwarfare by both states and non-state actors. Attacks on civilian and military nuclear facilities included extensive hacking in December 2020 of the US National Nuclear Security Administration which maintains US nuclear weapons . Complex global systems of early warning, command, control, communications, and intelligence are related to nuclear weapons. They are complex, dispersed, and interlinked—and vulnerable to cyberattack. As General James Cartwright, former head of US Strategic Command stated, it: "might be possible for terrorists to hack into Russian or American command and control systems and launch nuclear missiles, with a high probability of triggering a wider nuclear conflict."
British, French, Russian, and US authorities keep 2000 nuclear warheads on high alert, all mounted on delivery vehicles and ready for use within minutes of a launch order . These warheads are particularly vulnerable to digital sabotage and inadvertent or unauthorised launch. Many states, including China, Iran, Israel, North Korea, Russia, and the US, engage in offensive cyber operations . Buyers may include governments, government proxies, and terrorist organisations. Frequently buyers find tools in a lucrative global black and grey market offering hacking tools, especially 'Zero-day exploits'. These tools exploit software or hardware flaws and vulnerabilities for which no corrective patch yet exists . Government staff, as part of their work, or moonlighting staff, or government contractors can develop offensive digital tools. Individual or organised hackers and cybercriminals, or private for-profit companies can also produce them almost anywhere. Targets of hacking and digital sabotage to date include banking and health systems, Sony Corporation, electricity grids, water treatment facilities, airports, electoral systems, oil company computer systems, uranium enrichment centrifuges, and nuclear power plants. Increasing digital sophistication of nuclear weapons and delivery systems may increase their vulnerability to digital sabotage .
Source materials for nuclear weapons are not under adequate control
Vast stocks of fissile materials, the highly enriched uranium and plutonium from which nuclear weapons can be built, persist in civilian and military stockpiles in tens of countries. There are no effective international constraints on the production of these materials. Every state with a civilian nuclear industry is also capable of producing fissile materials; and any state that can enrich uranium to reactor grade can enrich it to weapons grade. Nuclear reactors inevitably convert some of the uranium in the fuel into plutonium. The average modern nuclear weapon contains around 4 kg of plutonium and/or 15 kg of highly enriched uranium (HEU) . With the global fissile material stockpile at the start of 2020 estimated by the International Panel on Fissile Materials to contain 1330 tonnes of HEU and 540 tonnes of separated plutonium,  this equates to more than 225,000 nuclear weapon equivalents of material . Apart from removal of relatively modest quantities of highly enriched uranium from civilian stockpiles in 34 countries plus Taiwan , the challenges of ceasing production of these materials, eliminating them where possible, and keeping the remaining quantities in consolidated storage in the safest possible form at the highest possible levels of security, remain largely unaddressed.
The TPNW provides our best path to control our worst weapons
The importance and urgency to eliminate nuclear weapons and to reduce the constant risk of their deliberate, inadvertent, or accidental use has never been greater. The Treaty on the Prohibition of Nuclear Weapons provides the most substantial positive development. It is firmly rooted in evidence of real consequences, costs, and dangers of nuclear weapons; it categorically and comprehensively prohibits these weapons; it contains the first treaty-based obligations for states to assist victims of nuclear weapons use and testing; and it obligates them to assist in remediation of environments so contaminated. This treaty is helping to drive divestment by responsible financial institutions from companies that profit from manufacturing the worst, and now illegal, weapons of mass destruction .
The treaty also contains the only internationally agreed and codified framework for the elimination of nuclear weapons. The treaty provides flexible pathways for nuclear-armed states to disarm before or after joining the treaty. It specifies plans for time-bound dismantlement of the weapons and the facilities that produce and maintain them. It is subject to verification by a competent international authority. Thus, the treaty provides the most promising pathway for all states to fulfil their obligations to negotiate in good faith to achieve nuclear disarmament. Many health and humanitarian organisations, including the World Federation of Public Health Associations, the Red Cross and Red Crescent movement, the World Medical Association, the International Council of Nurses, and the International Federation of Medical Student Associations have joined with International Physicians for the Prevention of Nuclear War (IPPNW) to welcome the entry into force of the treaty. These organisations urge all nations to join and faithfully implement it .